Narrow Results

False echoes degrade the operation of radar and imaging antennas. The false returns or clutter arise from antenna sidelobes and raise the threshold of detection in perimeter security systems. Accordingly there is great interest in reducing the sidelobes in present day millimeter wave and future terahertz antennas. Here we describe a new technique to suppress antenna sidelobe returns. The technique exploits our ability to distinguish between the phase of desired signals arriving in the antenna main beam and the phase of undesired clutter signals coming from the sidelobes. We demonstrate that through modulation of phase and taking the Fourier transform of the received signal, we can preferentially suppress the clutter return relative to the desired main beam signal. Suppression of average clutter return of over 25 dB is found.

The influence of extinction ratio (ER) on the performance of 120 Gbps return-to-zero octal differential phase-amplitude-shift keying (RZ-ODPASK) modulation format in single-channel transmission is investigated. Based on the modulation and demodulation schemes, the structures of RZ-ODPASK transmitter and receiver are presented; and expressions for the output optical fields and electrical currents of amplitude and phase tributaries are analytically acquired. As ER is a tradeoff for good performance of the two tributaries, the optimal ER for back-to-back configuration is theoretically derived as 2.32 dB for the first time, which is verified by simulation. Then the tolerances to residual chromatic dispersion (CD) and fiber nonlinearity of RZ-ODPASK transmission are numerically studied, respectively. The results indicate that phase tributary is more robust against both dispersion and nonlinearity impairment than the amplitude tributary. To balance the performance of the two tributaries, curves of the optimal ER values with the change of residual CD and fiber nonlinearity are also obtained, respectively. The results can be useful for the design of RZ-ODPASK systems; in the meantime, the analytical methodology described in this paper is applicable to other multilevel RZ-DPASK modulation formats.

We introduced a rigid structure into the acoustic metasurface design, the proposed labyrinth structure is based on the equivalent medium theory and different media are replaced by curly labyrinth. Layered media theory and equivalent medium theory are combined to design the arbitrary acoustic metasurface structure. An acoustic metasurface studied in this paper realized simultaneous phase modulation and energy attenuation in the air, the effective phase modulation range covered from 30${}^{\circ }$ to 90${}^{\circ }$ and the energy attenuation is over 40%. According to layered media theory which could modulate the acoustic wave direction, the metasurface with same function can also be applied to underwater case. Corresponding simulation results are calculated by FEA. Finally, by introducing the curly labyrinth theory, the underwater acoustic metasurface with simultaneous phase modulation and energy attenuation is designed and verified. This paper has potential applications in rigid underwater acoustic metasurface designs with low frequency, adjustable direction and sound energy attenuation.

We have considered soliton pulse compression in a dispersion decreasing ideal elliptical birefringent fiber with phase modulation. We have shown that there is exact balancing between the effective gain and the effective phase modulation and as a result arrive at the fundamental soliton solution. For the various dispersion decreasing profiles we have considered, we have shown that the hyperbolic profile is the best suitable choice for the dispersion decreasing profile as it provides fairly good compression even after compensating for the fiber loss.

Femtosecond coherent anti-Stokes Raman scattering (CARS) spectra from close Raman levels are indistinguishable due to the large bandwidth of the femtosecond laser pulse. In this paper, we propose a scheme to realize high resolution CARS spectra and further the selective suppression and excitation of the Raman levels by phase shaped femtosecond laser pulse. We show that the CARS signals can be narrowed and differentiated by tailoring only the probe pulse. In addition, one or more of the spectra peaks can be suppressed to realize the selective excitation by further scanning the pump pulse with $\pi$ or double reverse $\pi$ spectral phase modulation.

Different methods are used in the interferometer sensors for target signal extraction. Digital technologies provide new opportunities for precise signal detection. We have developed the principle of signal demodulation using an additional harmonic phase modulation and digital signal processing. The principle allows implementation of processing algorithms using different ratios between modulation and discretization frequencies. The expressions allowing calculation of the phase difference using the inverse trigonometric functions were derived. The method was realized in LabVIEW programming environment and was demonstrated for various signal shapes.

False echoes degrade the operation of radar and imaging antennas. The false returns or clutter arise from antenna sidelobes and raise the threshold of detection in perimeter security systems. Accordingly there is great interest in reducing the sidelobes in present day millimeter wave and future terahertz antennas. Here we describe a new technique to suppress antenna sidelobe returns. The technique exploits our ability to distinguish between the phase of desired signals arriving in the antenna main beam and the phase of undesired clutter signals coming from the sidelobes. We demonstrate that through modulation of phase and taking the Fourier transform of the received signal, we can preferentially suppress the clutter return relative to the desired main beam signal. Suppression of average clutter return of over 25 dB is found.